This invention relates in general to flat panel electronic fluorescent display devices and, in particular, to an improved flat matrix cathodoluminescent device with simplified multiple electrode structure and innovative processing methodologies particularly useful for large-area single piece full color hang-on-wall type displays.
Researchers in many flat panel display technologies, such as LCD, PDP, EL, LED, VFD, flat CRT, have been trying to develop a full-color hang-on-wall television. Color televisions of several-inch to slightly over 10-inch screens using LCD technology have been produced. Such televisions using LCD employ a large number of thin film transistors on their basic boards and are expensive. Because of difficulties and complexities in manufacturing these LCD displays, increasing the screen size of the LCD display is a formidable task and is very expensive. LCD displays employ a back illuminator scheme with color filters and polarizers. The base board using thin film transistors to control the front end light shutter transmits a low proportion of light from the backlight source and thus limits the brightness of the display. Because of these difficulties, research in the large-area (above 25-inch diagonal) display based on LCD technology has been primarily focused on projection display.
Full-color displays using Plasma Display Panel (PDP) technology have been limited to 40-inch screen size due to the complexity in fabrication of the discharge cells. In large-area full-color PDP displays, the main problems include the low efficiency in phosphorescence, low brightness, complicated IC driving circuitry, and the short product lifetime. Research in LED and EL displays have been in the development of a cost efficient luminescent material for emitting blue light. While multi-color displays have been developed using VFD technology, such devices are limited to smaller display screen sizes. Furthermore, except from the use of luminescent materials such as zinc oxide for generating blue-green light, the brightness, luminance efficiencies and product lifetimes of other color phosphors are not acceptable in the low operating voltage range of the VFD. From the above-mentioned shortcomings of these display technologies, it will be evident that large-area flat full color hang-on-wall displays that have been proposed using these existing flat panel display technologies are not entirely satisfactory.
Cathode ray tubes (CRT) have been widely used for display purposes such as consumer television systems because of its affordable costs. These tubes operate by scanning electron beams from a single electron gun. This conventional configuration inevitably adds depth to the dimensions of the device and limits it to small screen size. Thus, these CRT systems are bulky and are difficult to manufacture where the display screen size is larger than 40 inches. In many applications, it is preferable to use flat display systems in which the bulk of the display is much reduced. In U.S. Pat. No. 3,935,500 assigned to Oess et al., for example, a flat matrix CRT system has been proposed where a monolithic stack in which electron beams are formed and through which the beams are selectively projected onto a phosphor coated face plate. The stack structure has a number of holes through which electron beams may pass and sets of X-Y deflection electrodes are used to simultaneously control all the beams. The deflection control structure define by Oess et al. is commonly known as a mesh-type CRT structure. While the mesh-type flat CRT structure is simple in form, these structures are expensive to make, particularly in the case of large-area display systems.
Other conventional flat panel systems currently used include Jumbotron and Flatvision such as that described in Japanese Patent Publication Nos. 62-150638 and 62-52846, as well as in U.S. Pat. No. 4,955,581, respectively. The structures used in the Jumbotron and Flatvision display devices are somewhat similar to the flat matrix CRT described above. Each anode in the Jumbotron includes less thatn 20 pixels so that it is difficult to construct a high resolution display device with high phosphor dot density type display system using the Jumbotron structure.
The Flatvision having a shallow depth (.about.4.0 inches) consists of multiple sources of electron beams that are focused and aimed by a series of multiple electrodes arranged in sheetlike layers. Electrical charges are used to electrostatically deflect or aim the beams to hit the proper phosphor dots. Such flat CRT device requires precise alignment of the multiple grid electrodes to provide good image quality. Complex driving circuitry is required to control the passage of electron beams for scanning and data modulation.
The flat matrix CRT, Jumbotron and Flavision structures are somewhat similar in principle to the flat CRT system described by Oess et al. discussed above. These structures amount to no more than enclosing a number of individually controlled miniature electron guns within a panel, each gun equipped with its own grid electrodes for controlling the X-Y addressing and/or brightness of the display. In the above-described CRT devices, the control grid electrodes used are either in the form of mesh or perforated sheet structures. These mesh/sheet structures are typically constructed using photo-etching by etching holes in a conductive plate. The electron beams originating from the cathodes of the electron guns then pass through these holes in the mesh/sheet structures to reach a phosphor material on the anodes. As noted above, large-area mesh/sheet structures are difficult to handle in the manufacturing process of these display devices. Since the electron beam must pass through holes in the mesh/sheet structure, a large number of electrons originating from the cathode will not travel through the holes, but are lost to the solid part of the structure to become grid current such that only a small portion of the electrons will be able to escape through the holes and reach the phosphor material on the anode plate. For this reason, the osmotic coefficient, which is defined as the ratio of the area of the hole to the area of the mesh structure of the control electrode grid, of the above-described device is quite low.
As taught in U.S. Pat. No. 5,170,100 by Ge Shichao et al., to avoid the problem of low osmotic coefficient in conventional display devices, instead of using individually controlled electron guns, an electrofluorescent device (EFD) is proposed where two or more sets of elongated grid electrodes may be employed for scanning and controlling the brightness of pixels at the entire anode where the area of the grid electrodes that blocks electrons is much smaller than the area of the mesh structure of the conventional devices.
The above-described CRT-based devices have another drawback. In the case of the Jumbotron, each electron gun is used for scanning a total of 20 pixels. In the Oess et al. patent referenced above, each electron beam passing through a hole is also used for addressing and illuminating a large number of pixels. In the flat panel version, complex circuitry is required to drive a large number of electron beams for a 14-inch display device. When illumination at a particular pixel is desired, certain voltages are applied to the X-Y deflection electrodes on the inside surface of the hole, causing electrons in the electron beam passing through the hole to impinge the anode at such pixel. However, electrical noise and other environmental factors may cause the electron beam in the Oess et al. system, the Jumbotron and the Flatvision to deviate from its intended path. Furthermore, certain electrons will inevitably stray from the electron beam path and land in areas of the anode which are different from the pixel that is addressed. This causes pixels adjacent to the pixel that is addressed to become luminescent, causing crosstalk and degrades the performance of the display.
As is known to those skilled in the art, the inner chamber of a cathodoluminescent visual display device must be evacuated to high vacuum so that the electrons emitted by the cathode would not be hindered by air molecules and are free to reach the phosphor elements on the anode. For this reason, the housing that contains the cathode, anode and control electrodes must be strong enough to withstand atmospheric pressure when the chamber within the housing is evacuated to high vacuum. When the display device has a large surface area, as in large-screen displays, the force exerted by the atmosphere on the housing can be substantial especially when the chamber is evacuated to very high varuum (&lt;10.sup.-7 Torr). For this reason, conventional flat cathodoluminescent display devices have employed thick face and back plates to make a sturdy housing. Such thick plates cause the housing to be bulky so that the device is heavy, expensive and difficult for manufacture.
It is important in flat panel displays of the electronic fluorescent type that the spacer wall charging effect should be eliminated. There is a high voltage differential between the cathode and the display surface. The electrical breakdown between the electron emitting surface and the display surface must be prevented. Although numerous approaches have been used, the results were not very satisfactory especially for flat displays type where the spacing between the front light emitting surface and the back cathode has to be kept small. Eventhough the wall charging effect can be controlled using multiple electrodes to direct the passage of electrons, fabrication of multiple electrodes for such purpose requires precise control of spacing between each layer of the multiple electrodes and alignment of each electrode components for the electron to pass through. It is therefore desirable to provide an improved flat cathodoluminescent visual display device where the above-described difficulties are not present.